Field of the Invention
The present invention relates to an image forming apparatus, a method for controlling the image forming apparatus, and a program for controlling the image forming apparatus. More specifically, the present invention relates to an image forming apparatus for electrifying a photoreceptor with the use of an electrification member, a method for controlling the image forming apparatus, and a program for controlling the image forming apparatus.
Description of the Related Art
An electrophotographic image forming apparatus encompasses an MFP (Multi Function Peripheral) provided with a scanner function, a facsimile function, a copying function, a function of a printer, a data communication function, and a server function, a facsimile machine, a copying machine, and a printer.
An image forming apparatus generally forms an image on a sheet of paper by developing an electrostatic latent image formed on an image carrier to form a toner image, transferring this toner image to the sheet of paper, and then fixing the toner image onto the sheet of paper with the use of a fixing unit. Some image forming apparatuses develop an electrostatic latent image on a surface of a photoreceptor with the use of a developing device to form a toner image, transfer the toner image to an intermediate transfer belt with the use of a primary transfer roller, and secondarily transfer the toner image on the intermediate transfer belt to a sheet of paper with the use of a secondary transfer roller. In this case, the photoreceptor and the intermediate transfer belt serve as an image carrier.
A method for electrifying an electrophotographic image encompasses a corona discharge method and a contact discharge method. In those methods, the contact discharge method is an electrification method in which an electrification roller is brought into contact with a photoreceptor and a voltage is applied to the electrification roller to thereby perform proximity discharge and electrify a surface of the photoreceptor. The contact discharge method can advantageously reduce generation of an oxide (for example, ozone) caused by a high-voltage current flowing in the air.
Further, the contact discharge method encompasses a DC electrification method only using a DC (direct current) voltage as an electrification voltage to be applied to the electrification roller and an AC electrification method using a voltage in which an AC (alternating current) component is superimposed on a DC component as an electrification voltage to be applied to the electrification roller.
In the AC electrification method, discharge of electricity and removal of electricity generated between the electrification roller and the photoreceptor are forcibly repeated by an AC component. For this reason, the AC electrification method can advantageously have a higher electrification ability and higher uniformity of a potential on the surface of the electrified photoreceptor, as compared with the DC electrification method. The AC electrification method can also advantageously improve uniformity of development.
Note that the AC electrification method is required to have not only characteristics regarding an image but also performances such as an electrification characteristic and low noise. Generally, the electrification characteristic is a problem at a low frequency, and low noise and securing of durability of the photoreceptor are problems at a high frequency.
Meanwhile, in the AC electrification method, a periodic potential change occurs on the surface of the photoreceptor along a sheet-of-paper conveyance direction (rotation direction) depending upon an electrification frequency which is a frequency of an AC component. In the case where another factor (for example, image pattern) which periodically changes in the sheet-of-paper conveyance direction exists at the time of image formation, the periodic change caused by the AC component and the periodic change caused by the another factor interfere with each other, and this interference is visually recognized on an image as a fringe pattern (interference fringe) in some cases.
As a method for making the interference fringe inconspicuous, there is proposed a method in which a plurality of different values can be set as the electrification frequency and an electrification frequency having an appropriate value is selected in accordance with a printing condition. However, it is difficult to set a plurality of different values as the electrification frequency. Further, values that can be set as the electrification frequency need to be discrete values, and therefore values of the electrification frequency that can be set are limited in some cases.
A technique for reducing interference between a periodic change caused by an AC component and a periodic change caused by another factor is disclosed in, for example, JP 06-242663 A and JP 2008-152233 A. JP 06-242663 A discloses a technique that repeatedly performs an image forming process including a step of electrifying an image carrier by changing an electrification frequency and applying an oscillation voltage to an electrifier. In this technique, the electrification frequency is changed in each of the image forming processes that are repeatedly performed. JP 2008-152233 A discloses a technique that turns on/off superposition of an AC component of an electrification voltage during electrification and uses only a DC component as the electrification voltage.
Each developing roller and each electrification roller are grounded via a photoreceptor at the time of development, and therefore those rollers are electrically connected to each other via the photoreceptor. Thus, in some cases, beat noise is generated due to a shift between a frequency or phase of a developing voltage, which is a voltage applied to the developing roller, and a frequency or phase of an electrification voltage. The beat noise is a phenomenon in which voltage sags and swells are generated in a peak voltage of an AC component of a developing voltage or electrification voltage. In order to suppress generation of beat noise, the phase of the developing voltage is matched with the phase of the electrification voltage by setting a frequency of the AC component of the developing voltage to an integral multiple of the electrification frequency.
From this point of view, the following possibilities are not considered in the techniques of JP 06-242663 A and JP 2008-152233 A; a possibility that the phase of the electrification voltage is shifted by changing the electrification frequency during electrification; and a possibility that the phase of the electrification voltage is shifted by stopping superimposition of the AC component of the electrification voltage during electrification. Thus, in the techniques of JP 06-242663 A and JP 2008-152233 A, it is impossible to suppress generation of beat noise. Further, it is also impossible to suppress generation of an interference fringe caused by interference with a periodic change caused by another factor.
In addition, in the techniques of JP 06-242663 A and JP 2008-152233 A, it is difficult to control the electrification voltage. In the technique of JP 06-242663 A, it is necessary to switch the electrification frequency at a high speed to set the electrification frequency to a desired value while maintaining continuity of the phase of the electrification voltage before/after switching. In the technique of JP 2008-152233 A, it is necessary to control superimposition of the AC component of the electrification voltage with high accuracy so as not to distort the phase (waveform) of the electrification voltage.